Living organisms are autonomous chemical systems which include diverse sets of small molecules. Small molecules found in living systems include, for example, sugars, fatty acids, amino acids, nucleotides, and intermediates of metabolic and signaling pathways. Sugars are a primary source of chemical energy for cells. The cells break the sugars down through a series of oxidative reactions to small sugar derivatives and, ultimately, CO2 and H2O. Fatty acids are used for both energy storage and as major components of cellular membranes. Amino acids are the building blocks of proteins. Nucleotides are involved in intracellular signaling, energy transfer, and as the monomers of the information macromolecules, RNA and DNA.
The cellular small molecules are, generally, composed of six elements (C, H, N, O, P, S). If water is excluded, carbon compounds comprise a large majority of the cellular small molecules. The cellular small molecules repeatedly use certain distinctive chemical groups, such as methyl (CH3), carboxyl (COOH) and amino (NH2) groups.
In recent years, scientists have attempted to study cells and living systems through the cataloging of the entire genome of organisms through genomics, and the entire proteinome through proteomics. Metabolomics is the study of the small molecules present in a particular organism or a portion there of. Metabolomics has the potential to complement proteomics and genomics, as well as make an independent contribution to the global understanding of systems biology, from individual cells to entire populations of organisms.